Description of problem: After attaching a second monitor to my notebook the desktop it successfully extended but the gnome shell behaves very slow and is actually unusable because a mouseclick is processed after more than ten seconds. /usr/bin/top shows that gnome-shell uses more than 50% of cpu (normally 1%). After disconnecting the second monitor, gnome-shell behaves well. Version-Release number of selected component (if applicable): gnome-shell-3.0.1-4.fc15.i686 xorg-x11-drv-intel-2.15.0-3.fc15.i686 How reproducible: Just attach a second monitor to your Fedora 15 notebook and try it. Expected results: The desktop should be usable with the second monitor.
Are you by chance running on i945 hardware? If so, attaching second monitor could make your desktop extend beyond 2048 pixels and run into hardware limits. If you have i945 hardware, I suggest stacking your monitor vertically (to be within 2048 limit). You can also try to boot with second monitor attached. You should be taken straight into Fallback mode.